COURSE OF PRACTICAL CHEMISTRY. PART I. CHAPTER I. CHEMICAL AND PHYSICAL CHANGE. Exp. 1. Hold a piece of platinum wire in the flame of a Bunsen-lamp for a few minutes. Then hold a piece of magnesium ribbon in the same flame. The platinum becomes red-hot, glows, and emits light; when removed from the flame, it presents the same appearance as before it was heated. The magnesium also glows; but in addition to this, it is burnt: a white powder is produced, unlike the magnesium; this white powder is called magnesia. The change produced in the platinum was a physical change. The change of magnesium to magnesia was a chemical change. Exp. 2. Magnetise a knife-blade, by drawing the poles of a horseshoe magnet over it several times in the same direction. Bring the magnetised blade close to a quantity of iron-filings; the iron is attracted to the blade. The steel which forms the blade has acquired a new property, but it still exhibits all those properties which characterise steel. Place some iron-filings in a porcelain basin, and add some dilute sulphuric acid; when effervescence has ceased, add a little more acid, then evaporate until the liquid becomes slightly thick, but take care that the whole of the iron has not M. P. C. 1 disappeared; filter while hot, and allow the greenish coloured liquid which runs through the filter-the filtrate-to cool. Green crystals of sulphate of iron are formed as the liquid cools. Compare these crystals with the iron-filings with which you began the experiment, as regards colour, appearance, hardness, and solubility or insolubility in water; the green crystals are evidently quite a different kind of matter from the iron. The change produced in the knife-blade was a physical change. The change of the iron to sulphate of iron, by causing it to interact with sulphuric acid, was a chemical change. By the process of filtration, a liquid is separated from a solid. The liquid which runs through the filter is called a filtrate. Exp. 3. Heat a little iodine in a large dry test tube, heating only that part of the tube where the iodine is. The iodine slowly changes to a dark violet vapour, but as this comes into contact with the colder parts of the tube, a solid body is formed; this solid is easily seen to be the same as the iodine before it was heated. The change of a solid to a gas, followed by the re-formation of the solid on cooling the gas, is called sublimation. Heat a little dry powdered lead nitrate in a dry test tube. A reddish-brown gas, called nitrogen tetroxide, is formed, and a yellowish-white solid, called lead oxide, remains. The action of heat on the iodine has been to produce a physical change. The change of lead nitrate to lead oxide and nitrogen tetroxide, produced by the action of heat, has been a chemical change. Exp. 4. Dissolve a little common salt in water,* in a porcelain basin; place the basin on the top of a beaker, of a size such that part of the basin is within the beaker; put a piece of paper between the beaker and basin; put some hot water in the beaker and place it over a lamp. (Fig. 1.) As the water boils the steam surrounds the basin and heats its contents nearly to 100°. The water in the basin is thus evaporated, without danger of the solid matter, which is formed as the water passes away, being lost by spirting. When the contents of the basin are dry, collect some of the D Fig. 1. * Always use distilled water. white solid and compare its properties with those of the common salt you used; note the colour, appearance, taste, and solubility in water, of each. In all cases where evaporation of a liquid is to be continued until the liquid is wholly removed, the final stages of the process should be conducted on a water-bath (as described above), unless special directions are given to the contrary. The salt has been changed by the action of water; it disappeared in the water; but by removing the water the salt has been again obtained. The change has been physical, not chemical. Place a little water in a basin, and into it throw one or two small pieces of a white, soft, lustrous, solid called sodium. Never touch sodium with wet fingers. The sodium slowly disappears in the water with a hissing noise; when the sodium has all gone, place the basin on a water-bath and evaporate until the water is wholly removed. This is called evaporating to dryness. You obtain a white, hard, lustreless, solid, very unlike the sodium thrown into the water. This solid is called caustic soda or sodium hydroxide. The change of sodium to caustic soda is a chemical change. Exp. 5. Mix a little blue coloured solution of litmus with some colourless water. The result is a liquid coloured lighter blue than the litmus; the properties of this liquid are those of the litmus added to those of the water. Mix the two colourless liquids, solution in water of potassium iodide and solution in water of mercury chloride. A reddish-yellow solid is at once formed, called mercury iodide. The appearance and colour of this shew that its properties are different from those of either of the bodies by the interaction of which it has been produced; it is evidently a different kind of matter from either of these. A solid substance formed by the interaction of one liquid with another, or of a gas with a liquid, is generally called a precipitate. The mercury iodide is the precipitate in the foregoing experiment. We shall use the contraction pp. for precipitate. The litmus and the water were physically changed: no new kind of matter was formed. The mercury chloride and the potassium iodide solutions were chemically changed: a new kind of matter, mercury iodide, was formed. |